The bacterial flora in the human colon, although extremely diverse, has a relatively stable composition and non-infectious anaerobic bacteria are dominant. The flora forms a pool of numerous different antigens separated from mucosal immunocompetent cells by just a single layer of epithelial cells. Despite this thin barrier, however, the colonic mucosa is physiologically only mildly inflamed. This study looked at the mucosal humoral immune response against faecal anaerobes. By flow cytometric analysis the in vivo immunoglobulin coating of anaerobic bacteria in faecal samples of 22 healthy human volunteers was determined. In a previous study flow cytometric analysis of faecal bacteria has been found to be a very sensitive method to detect immunoglobulins on faecal bacteria. This technique showed that in vivo many bacteria are coated with IgA (24-74%) and less with IgG and IgM. The presence of many bacteria coated with IgA implies that IgA coating does not result in permanent removal of the species from the colon. The absence of immunoglobulin coating suggests that there is immunological unresponsiveness for anaerobic bacterial antigens. It is concluded that both immunological unresponsiveness and preferential coating with IgA are responsible for the relative absence of colonic mucosal inflammation.
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